1. Field of the Invention
The present invention relates generally to a procedure for inserting pedicle screws into the spine as part of a spinal fixation instrument system, and particularly to a procedure whereby the likelihood of nerve damage caused by improperly placed pedicle screws can be reduced by implementing the present procedure with associated apparatus in situ.
2. Discussion of the Known Art
Instances arise when it becomes necessary to stabilize or fuse a portion of the spine from motion such as, for example, (1) after decompression wherein certain posterior spinal elements are removed to relieve pressure on neural elements, (2) after trauma or (3) because of the presence of tumors. Instrument systems that accomplish spinal fixation are known in the form of pedicle screws which are adapted to be inserted in selected vertebrae, and stiff rods or plates that connect adjacent pedicle screw heads to one another after the screws are inserted, thus resulting in the fixing or bracing of all vertebrae spanned by the rod or plate. Commercially available pedicle screws are usually made of stainless steel having overall diameters (including threads) ranging between 5.5 mm and 6.5 mm, and with lengths ranging between 25 mm and 55 mm.
The pedicles are the strongest parts of the spinal vertebrae and thus provide a secure foundation for the screws to which fixing rods or plates are attached. See R. Roy-Camille, et al, Internal Fixation of the Lumbar Spine With Pedicle Screw Plating, Clinical Orthopedics (Feb. 1986), at page 7; and H. N. Herkowitz, et al, Instrumentation of the Lumbar Spine for Degenerative Disorders, Operative Techniques in Orthopaedics (Jan. 1991), at page 91.
In order to derive the greatest mechanical integrity when anchoring pedicle screws in a spine fixing instrument system, it is therefore essential that the screws be guided and threaded in alignment with the pedicle axis and not be allowed to deviate off axis in which case the screw body or its threads will break through the vertebral cortex and impinge on or become dangerously close to surrounding nerve roots. A jig adapted for providing locations on the pedicles of a vertebra for insertion of pedicle screws, is disclosed in U.S. Pat. No. 4,907,577 (Mar. 13, 1990). The patent observes that the vertebral bodies will be fixed more stably the deeper the screws are inserted in the pedicle, and that slight deviations in the angle of screw insertion can injure nerve roots or the spinal cord.
Much appears in the literature with respect to the problems of misalignment of pedicle screws and the symptoms arising when the screws make contact with neural elements after breaking outside the pedicle cortex. Cutting into a nerve root or simply contacting the root gives rise to various postoperative symptoms such as dropped foot, neurological lesions, sensory deficits, or pain. The Adult Spine--Principles and Practice, Vol. II, at pages 1937 and 2035-36 (Raven Press 1991); J. L. West, et al, Complications of the Variable Screw Plate Pedicle Screw Fixation, Spine (May 1991), at 576-79; and J. L. West, et al, Results of Spinal Arthrodesis with Pedicle Screw-Plate Fixation, Journal of Bone and Joint Surgery (Sept. 1991), at 1182-83.
Apart from the jig of the mentioned '577 patent, no tools or devices are known with which pedicle screws can be guided or inserted into a vertebra in such a manner as to ensure that parts of the screws do not rupture the cortex and impinge on or come dangerously close to nerve roots. Current practice calls for the use of recognized landmarks along the spinal column for purposes of locating pedicle entry points, and the use of X-ray exposures or fluoroscopy to monitor the advancement of a metallic pedicle screw through the vertebra. But prolonged radiation exposure of the patient for purposes of proper screw placement is of course undesirable and this technique still has a misplacement rate of up to 21%. The Adult Spine--Principles and Practice, supra, at 2035. Nonetheless, a disturbingly high number of reported pedicle cortical disruptions has prompted one author to observe that "[a]lthough pedicle disruption does not necessary cause neural deficit, keeping the screw contained within the pedicle is one sure way to prevent it." The Adult Spine--Principles and Practice, supra, at 1937.
It is known generally that electrical potential pulses may be applied on or into the body of a patient for purposes of treatment. For example, U.S. Pat. No. 4,461,300 (Jul. 24, 1984) discloses a specially formed electrode for healing of bone or soft tissue fractures in a patient. The electrode has a lead wire connected at its back end, and is capable of being drilled or otherwise inserted into the patient's body with the lead wire in place.
It is also known that muscular reactions to electrical stimulation can be observed while a procedure is continued in accordance with the observed reactions. U.S. Pat. No. 2,808,826 (Oct. 8, 1957) shows electro-diagnostic apparatus and associated circuitry that act as a stimulator to measure the excitability of muscle or nerve tissue. A pair of electrodes are placed across a part of the patient's body and short duration pulses are applied with the pulse amplitude being slowly increased until a visible contraction appears. Electrical current readings are obtained for pulses of increasing duration, and a curve called a "strength-duration" curve is obtained, according to the patent. U.S. Pat. No. 4,824,433 (Apr. 25, 1989) discloses a puncturing and catheterizing device with a metal puncture needle and cannula suitable for puncturing nerve tracts. With the device connected to an electrical pulser, as long as the needle and surrounding cannula are inserted in the body through a nerve sheath, current pulses applied to the device induce visible motor reactions on body parts such as the hand. The visible reactions allow the physician to know that the puncture needle and cannula are being passed correctly along a space between a nerve and the nerve sheath.
Until applicant's invention, no technique, procedure or device was known that utilized visible motor reactions of a patient's limbs as a means for determining if a pedicle screw to be inserted in the patient's spine might impinge on or come in dangerously close proximity to a nerve root.